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Table 2 Examples of restricted randomization descriptions from C-RCTs published in high impact journals

From: Allocation techniques for balance at baseline in cluster randomized trials: a methodological review

Matching

‘To help ensure comparability of the intervention and comparison communities with respect to baseline HIV and STD prevalence and risk factors for infection, the communities were matched into six pairs according to the following criteria: roadside, lakeshore, island, or rural location; geographical area (paired communities were generally in the same district and less than 50 km apart); and prior STD attendance rates at the health centre. In each matched pair, one community was randomly chosen to receive the STD intervention’ [32].

Stratification

‘To ensure balance between the 2 study arms, family physician practices underwent stratified randomization on the basis of the mean age (< 65 v. ≥ 65 years) and annual rates of emergency department visits (< 200 v. ≥ 200) of their clientele. Stratified randomization was achieved by a separate randomization procedure performed within each of the strata’ [33].

Minimization

‘We randomized practices to intervention and control groups using a minimization programme, stratifying by partnership size, training practice status, hospital admission rate for asthma, employment of practice nurse, and whether the practice nurse was trained in asthma care’ [34].

Covariate-constrained randomization

‘A balanced randomization procedure ensured that the intervention and control hospitals were balanced with respect to the rates of prophylactic use of oxytocin and episiotomy, the presence or absence of residency programs, the country and region where the hospital was located, and the annual number of births at the hospital. Of 184,756 possible ways of assigning hospitals to the intervention and control groups with acceptable balance, one sequence was randomly selected to determine the composition of the two groups’ [35].